1. Field of the Invention
The present invention relates to electronic devices, and more particularly to wireless sensors fabricated by thermal spray technology.
2. Discussion of the Prior Art
Sensors are ubiquitous in modern engineering systems. They are used to measure temperature, strain, pressure, humidity, etc., in a vast array of materials. One of the ongoing challenges in many sensor applications, however, is how to extract the signal from the sensor for recording and further processing. This issue is particularly challenging for situations immersed in harsh environments, including, for example, those containing high temperatures, corrosive materials, significant wear situations, significant and/or prolonged vibration, thermal loading, etc. Practical examples of harsh environments include the hot section of a gas turbine or an internal combustion engine, boilers for power and steam generation, power transmission components, etc.
Sensor signal extraction can be considered in two classes, wire-based and wireless. Wire-based sensors represent a large and diverse class of applications in which electrical wires are physically connected to the sensor or sensor system, and a signal is delivered to a point of interest. In many situations, such an approach is adequate. For harsh environments and environments in which the geometry precludes the convenient connection of physical wires, wireless approaches have been developed. Wireless sensors transmit the sensor data using electromagnetic, optical, acoustical, or other means of information transmittal to a suitable receiver. Electromagnetic (EM) wireless sensors represent a significant portion of wireless signal extraction techniques.
Wireless EM sensors, referred to in the following as wireless sensors, can be further classified as active or passive. Active wireless sensor systems modulate an EM signal in response to the sensor output and need some form of power supply in the sensor circuit itself to provide the energy needed to relay the sensor information to a suitable receiver. Such systems suffer from several significant drawbacks: they need either periodic replacement of the energy source, e.g., a battery, or some form of renewable energy source, which can be expensive, adds complexity, and can be unreliable. Further, they almost invariably incorporate silicon-based active electronics, which need additional measures for protection in harsh environments while also providing limits on the temperatures at which the devices can be used. Active wireless systems are typically large and bulky, and thus require additional space, special provisions for sensor, electronic, and power supply attachment, and can be difficult to integrate directly into functional components.
Passive wireless sensors, on the other hand, derive the energy needed for signal transmission from an outside source. Variations of this technique include 1) modulating an externally applied source signal, 2) temporarily storing the externally applied energy in a storage medium, after which it is used to transmit the signal, and 3) converting one form of energy into another. Passive wireless systems that temporarily store energy to power active electronics suffer from the same drawbacks as their active counterparts discussed above.
U.S. Pat. No. 6,254,548 by Ishikawa et al. represents an example of an active wireless sensor that needs no battery. It teachers of a small spherical-shaped wireless temperature transponder utilizing active electronics that are powered by converting an externally applied EM field into electrical energy suitable to drive the encapsulated electronics. The sensor, however, is active, and utilizes silicon-based components.
U.S. Pat. No. 6,113,553 by Chubbuck teaches of a passive wireless sensor to measure intracranial pressure. The sensor element is embedded within the skull of the patient and an external probe is placed on the other side of the skull to record the pressure. A bellows in the sensor deforms according to the imposed pressure and alters the resonance frequency of the circuit, which is recorded by the external probe. While providing a passive sensor system, the probe assembly is bulky, complicated, needs considerable labor and expense to fabricate, and relies on mechanical deformation of the bellows (which can be unreliable and prone to failure), and needs the receiver (the probe) to be placed within a few millimeters of the sensor for accurate readout.
U.S. Pat. No. 5,818,340 by Yanklelun and Flanders discloses a passive wireless sensor system to measure moisture in the roofs of structural buildings. An inductor-capacitor resonant circuit is formed in which the capacitor is formed by two concentric plates with a moisture-sensitive dielectric between said plates. Variations in moisture result in variations in capacitance, which shift the resonant frequency of the circuit. An external antenna provides an RF pulse to an array of such sensors on a rooftop, and then records the resonant frequencies of the sensor systems. Moisture content is detected by a shift in resonant frequency from that of the dry sensors.
U.S. Pat. No. 5,278,442 by Prinz, Weiss, and Siewiorek teaches the use of thermal spray to form electronic packages and smart structures, including strain gauges and thermocouples. The thermal spray method of Prinz is complex, cumbersome, slow, and limited in feature size because it relies on a series of masks to selectively deposit material in a multi-layer fashion.
To employ these technologies, the sensors need to be fabricated in a separate manufacturing step at a specialized facility for such devices, followed by the attachment of the sensor device to the surface or component of interest, typically after the component has been manufactured. For RF-based systems, antennas need to be added, adding additional cost, time, and labor. The addition of the sensing system in many cases degrades the performance of the component, for example, thermocouples that need to be cemented to surfaces for fluid flow can result in flow disturbance and turbulence formation as a result of the added devices.
Electronic manufacturing with feature sizes in the meso-scale regime (e.g., in the range of about 10 to 1000 micrometers) typically needs multi-step processes that include time-consuming photolithographic methodologies. The time needed between iterations is typically measured in terms of weeks. In addition, thick film electronics based on the ceramic multi-chip module technology, including low temperature co-fired ceramic modules (LTCC-M) and high temperature co-fired ceramic modules (HTCC-M) need firing of the screen printed pastes to moderate ˜800 C for LTCC-M or high 1400 C for HTCC-M. The high temperature curing process sets-up issues associated with mismatch in thermal expansion between dissimilar materials and can lead to premature debonding. This needs to be accounted for during the processing through careful tailoring of the properties of the layered materials. Current screen printing technology is inherently limited in its fine feature capabilities; the line width being limited to 100 microns or higher.
Therefore, a need exists for a method of fabricating wireless sensors on functional engineering components and pre-existing surfaces.